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Geological Terms Beginning With "G"



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Gabbro

Gabbro

A black, coarse-grained intrusive igneous rock that is the compositional equivalent of basalt. Composed of calcium-rich feldspars, pyroxene and possibly olivine, but containing little if any quartz.

Gage Height

Gage Height

A measured height of water above an arbitrary reference datum. Frequently used to describe the height of water in a stream, lake, well, canal or other water body. Stage and gage height are equivalent words, with stage being more commonly used by the public. Gage height is usually measured at a gaging station.

Gaging Station

Gaging Station

A facility on a stream, lake, canal, reservoir or other water body where instruments are installed to automatically monitor the water. Measurments such as stage, discharge, water temperature and pH are automatically taken and transmitted to hydrologists via satellite, radio or telephone. Measurements from these stations are useful for a wide variety of flood prediction, water management, recreation and navigation purposes.

Garnet

Garnet

Garnet is the name of a group of silicate minerals that share a common crystal structure but they vary in composition. Most garnets are red in color, but the mineral also occurs in orange, yellow, purple, green, pink, black, and other colors. Shown in the photo from top to bottom are the gem garnets: spessartine, almandine, mali, rhodolite and tsavorite. In addition to being used as a gemstone, garnet is used as an abrasive, filter medium, sand-blasting granule and waterjet cutting granule.

Gas Field

Gas Field

The geographic area that is directly above an underground accumulation of natural gas that is commercially viable. The image shows the geographic extent of the Barnett, Haynesville and Eagle Ford gas fields.

Gathering System

Gathering System

A network of small pipelines that connect producing wells to the main transmission system.

Dictionary of Geological Terms   Dictionary of Geological Terms

All scientific disciplines have an essential vocabulary that students and professionals must understand to learn and communicate effectively. A geology dictionary that is used reguarly is one of the most important tools for developing professional competence. A good dictionary should be on the desk of every geologist and within easy reach. This dictionary is compact and inexpensive at only $18.95. More information.
gemstone

Gemstone

There is no universally agreed upon definition for the word "gemstone". The word usually triggers an image of attractive mineral materials that have been fashioned into gems to be worn for personal adornment. Some people believe that gemstones should also be durable, rare, valuable and skillfully cut - but some items formally called "gemstones" lack all of these. Many gemstones are rocks, organic materials, mineraloids or even objects from space that lack durability, rarity, intrinsic value and are used in their natural state. As an example consider a small pearl used in an inexpensive necklace sold at Walmart. Hundreds of materials have been used as gemstones, see photos of over 100 here.

Geologic Column

Geologic Column

A diagram that shows the vertical sequence of rock units present beneath a given location with the oldest at the bottom and youngest at the top. They are typically drawn to approximate scale with proportional rock unit thicknesses. Colors and standardized symbols are usually added to graphically communicate rock types and some of their more important features. Geologic columns prepared for regions will have generalized thicknesses and rock unit features that show relationships that change over distance.

geology

Geology

Geology is the study of the Earth, the materials of which it is made, the structure of those materials, and the processes acting upon them. It includes the study of organisms that have inhabited our planet. An important part of geology is the study of how Earth’s materials, structures, processes and organisms have changed over time.

Geomorphology

Geomorphology

The study of Earth's surface, including the origin, description and classification of landforms such as mountains, valleys, drainage systems, coastlines and ocean basins. It includes the processes that form them and how they are influenced by the underlying bedrock.

geotechnical

Geotechnical

Refers to the use of geology as a science when applied to an engineering problem such as landfill design, highway construction, landslide repair, tunnel construction, sewage system design and much more.

Geothermal Gradient

Geothermal Gradient

The progressive increase of temperature with depth into the Earth. Often displayed graphically as a chart similar to the image at left which shows the geothermal gradient in the Carson Sink area of Nevada from a USGS report.

Geyser

Geyser

A hot spring that intermittently erupts a spray of steam and hot water. Caused by the heating of groundwater within a confined opening in hot rock.

Glacial Rebound

Glacial Rebound

A very gradual uplift of Earth's crust that occurs after the weight of a thick continental ice sheet (which produced subsidence) has melted away.

Glacial Striations

Glacial Striations

Grooves and scratches on a bedrock surface that were produced by the movement of a glacier. The orientation of the striations gives evidence to the direction of glacial movement.

Glacial Valley

Glacial Valley

A valley with a U-shaped cross section that was cut by an alpine glacier.

Glacial Valley

Glacier

A thick mass of ice that forms on land from an accumulation and recrystallization of snow significant enough to persist through the summer and grow year by year. There are two basic types of glaciers: 1) valley (or alpine) glaciers that creep downslope under the influence of gravity, and 2) continental glaciers that flow outward from a thick central area under their own weight.

Glass

Glass

An amorphous (without crystal structure) igneous rock that forms from very rapid cooling of magma. The rapid cooling does not provide enough time for crystal growth. Shown in the image is a piece of obsidian.

Gneiss

Gneiss

A coarse-grained, foliated rock produced by regional metamorphism. The mineral grains within gneiss are elongated due to pressure and the rock has a compositional banding due to chemical activity.

gold dust

Gold Dust

Fine particles of native gold that have been weathered out of their host rock. They can be flake, nugget or wire-shaped particles of gold. They can be mined from a placer deposit or milled from the rock of a lode. Image © Gilles_Paire, iStockphoto.

gold nuggets

Gold Nugget

A piece native gold that has been weathered out of its host rock. Nuggets are found in placer deposits downslope from a lode. They might be found in soils, stream sediments or beach sediments. Nuggets are often smoothed and rounded, which is evidence of transport. They sometimes still contain pieces of host rock. They are generally not pure gold, instead being natural alloys with silver or copper that are 80% to 95% gold. Image © Goruppa, iStockphoto.

gold pan

Gold Pan

A broad, shallow pan made of metal or rigid plastic that is used to separate the lighter fraction of a sediment from heavier grains. A shovel of stream sediment or soil is placed in the pan, the rocks are picked out and clinging soil or sediment scraped off, then the pan and sediment are immersed in the stream and moved in a manner that allows lighter grains to be removed by the current or sloshed over the rim of the pan. Considerable practice is required but an experienced person can separate sand, silt and mud from particles of gold or heavy mineral that are so small that they can barely be seen. View a gold panning demonstration. View gold panning supplies in the Geology.com Store.

Graben

Graben

An elongated, downthrown block bounded by two normal faults that dip steeply in opposite directions. Produced in an area of crustal extension. This is the dominant structural style of the Basin and Range province of the southwestern United States. Death Valley, Salt Lake Valley and Owens Valley are all grabens in that province.

Graded Bedding

Graded Bedding

A rock or sediment layer that has a progressive change in particle size from top to bottom. Most common is a sequence with coarse grains at the bottom and fining upwards, which is typically caused by a declining current velocity within the depositional environment.

Granite

Granite

A coarse-grained, intrusive igneous rock composed primarily of light colored minerals such as quartz, orthoclase, sodium plagioclase and muscovite mica. Granite is thought to be one of the main components of continental crust. Granite is also used as a dimension stone for kitchen countertops, building stone, paving stone, tile, memorials, facing stone, curbing and many other uses.

Granule

Granule

A term used for a sedimentary particle that is between 2 and 4 millimeters in size. Granules are larger than sand but smaller than pebbles. Granules have typically been rounded by abrasion during sedimentary transport.

Gravel

Gravel

Clastic sedimentary particles of any composition that are rounded and over 2 mm in diameter. Includes granules, pebbles, cobbles and boulders. If lithified, an accumulation of gravel would produce the sedimentary rock known as conglomerate. The image shows an accumulation of gravel on the surface of Mars discovered by Mars Rover Curiosity in 2012. The largest particle in the photo is about one centimeter in length.

Gravity Anomaly

Gravity Anomaly

A geographic area where observed gravity values depart from those of the assumed Earth model. They are typically a response to lateral density differences within the Earth caused by variations in geologic structure or crustal composition. The image is a Bouguer Gravity Anomaly map of the state of Ohio.

Greenhouse Effect

Greenhouse Effect

A warming of the atmosphere caused by carbon dioxide and water vapor in the lower portions of the atmosphere capturing heat that is radiated from and reflected by Earth's surface.

Greenstone

Greenstone

A low-grade metamorphic rock that frequently contains green minerals such as chlorite, epidote and talc, often derived from the metamorphism of basalt, gabbro or diabase.

Ground Moraine

Ground Moraine

A blanket of till that is deposited during the retreat of a glacier, producing a rocky ground cover made up of material that ranges in size from clay to boulders. Much of this debris was carried beneath the glacier but some was contained within the ice and released by melting.

Ground Water

Groundwater or Ground Water

Water that exists below the water table in the zone of saturation. Groundwater moves slowly in the same direction that the water table slopes.

The American Geosciences Institute's Glossary of Geology 5th Edition has an entry for "groundwater" but none for "ground water". The National Ground Water Association uses "ground water" in their name but their mission statement includes "enhance the skills and credibility of all groundwater professionals". Their reports use "groundwater" with great frequency. Another organization is "The Groundwater Foundation" and they use "groundwater" regularly in their published information. The United States Geological Survey and the United States Environmental Protection Agency use "groundwater" and "ground water" inconsistently in their reports.

A few people have written to Geology.com (usually from USGS and EPA) to scold and argue about how this term is used on our website (we use "groundwater" almost everywhere). However if you search the EPA website for instances of "ground water" or "groundwater" you will see that each of these terms appears on hundreds of thousands of pages. The USGS website uses "groundwater" and "ground water" on nearly 100,000 pages each. These numbers are as of December, 2014. So, please check your own website before writing. If your agency has made a decision on how this word should be used and your website is consistent, let us know.

Ground Water Recharge Area

Groundwater Recharge Area

A location where surface water or precipitation can infiltrate into the ground and replenish the water supply of an aquifer.

Guyot

Guyot

A seamount with a flat top. They are usually shield volcanoes that have a flat top produced by wave erosion. Shown in the image is the Bear Seamount of the North Atlantic Ocean about 200 miles east of Woods Hole, Massachusetts.

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